The present invention relates to improved fixed bed attrition resistant catalysts and to their use in exothermic chemical processes, such as gas phase oxidation of acrolein to acrylic acid.
Catalysts useful for exothermic reactions have been prepared conventionally as pills or tablets having essentially a uniform catalyst composition throughout. Catalysts prepared in this manner can be extremely active when used in fixed bed exothermic oxidation reactions. In some cases however, "hot spots" can cause large amounts of undesirable by-products to be produced, due to the fact that the heat generated by the reaction cannot be dissipated efficiently by normal heat transfer techniques.
One method which has been tried to overcome this difficulty is to use inert catalysts supports which contain the catalytically active materials coated on the support surface. This reduces the amount of catalytically active material in the reactor and cuts down undesirable catalytic activity, thereby better controlling the heat produced in the reaction and substantially eliminating "hot spots". However, such coated catalysts often exhibit a significant degree of attritability, i.e., they break down in use to give significant amounts of catalyst fines. As these fine particles accumulate in the reactor, high pressure drops develop. Once the pressure reaches an unacceptably high level, the reaction must be stopped even though the catalyst may still retain good activity.
Attempts have been made in the past to provide active supported catalysts having acceptable attrition resistance. For example, U.S. Pat. No. 3,341,471 describes attrition resistant solid catalysts prepared by mixing finely divided catalytic active components with an aqueous silica sol, forming a paste or slurry, drying the mixture, heat-treating the finished finely divided catalyst and then pelletizing, yielding a catalyst having catalytically active material essentially uniformly distributed throughout the pellets. This catalyst, however, when used in a fixed bed reactor in the oxidation reaction of olefins to oxygenated hydrocarbons, among other reactions, still has a tendency to result in undesirable "hot spots".
U.S. Pat. No. 4,077,912 describes a procedure for producing a coated catalyst said to have good attrition resistance which involves wetting a porous inert support with "an inorganic or organic liquid", water and petroleum ether being specifically disclosed, in such a manner that the resulting wetted support does not have the appearance of free liquid in contact with the support particles, then gently agitating a catalytically active material onto the surface. This patent also identifies many other U.S. patents which describe various techniques of producing supported catalysts. U.S. Pat. No. 3,644,059 describes the catalytically active oxide material which can be utilized in producing the catalysts of the present invention.
None of the aforementioned patents describe the attrition resistant catalysts of the present invention, nor do they describe the present inventive method of preparing such catalysts.